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Subject: Re: [boost] [next gen future-promise] What to callthemonadicreturntype?
From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2015-05-26 08:31:25

On 26 May 2015 11:54 am, "Peter Dimov" <lists_at_[hidden]> wrote:
> Giovanni Piero Deretta wrote:
>> On 26 May 2015 12:59 am, "Peter Dimov" <lists_at_[hidden]> wrote:
>> > This non-allocating implementation is an interesting argument in favor
> of the current "unique future", which I've long disliked. I prefer >
futures to be shared_futures.
>> Interesting, why do you dislike the unique future design?
> I dislike the unique/shared future split, which requires all algorithms
to be duplicated/manyplicated.
> auto f = when_any( f1, f2, f3, f4 ); // 2^4 options

The algorithm can be generic of course. You fear the template instantiation
explosion? You'll have the same problem if you want to mix different
futures from separate libraries.

>> A shared future pretty much requires holding a shared pointer and needs
heavy weight synchronisation ( a muted+condvar or equivalent). On the other
hand a unique future need no internal mutual exclusion and the only
synchronisation is needed for the handoff between producer and consumer;
the reference count is implicit (just have the consumer always deallocate
the object). The implementation can be significantly more light weight.
> I already acknowledged that these implementations are an argument in
favor of unique future.
> But I don't think I agree with what you're saying. I don't see how
mutex+condvar is required by shared future but not by unique future (how do
you implement wait() is not related to sharedness), neither do I see how
unique futures require no synchronization (they obviously do in Niall's

You are right of course. The readiness notification is orthogonal, but in a
shared future you need a mutex to synchronise the various consumers access
to the shared state, so a condition variable becomes the obvious choice,
while you can be more creative with plain futures.

Synchronisation with unique futures is of course necessary, but is much
simpler: the producer will set the future to ready exactly once, then never
touches it again, while the consumer won't access the future state until it
is ready. Basically the producer need a single strong CAS on an atomic
object to see the readiness and check for waiters, similarly the waiter
need a CAS to test and wait. No explicit mutual exclusion is necessary.

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